1. Technical Field
The present invention relates to a semiconductor device including a bipolar transistor.
2. Related Art
FIG. 8 shows a configuration of a semiconductor device 200 of a conventional bipolar transistor.
The semiconductor device 200 includes: a semiconductor layer 206 including a p− type semiconductor substrate 202 and an n− type epitaxial layer 204; an n+ type impurity buried layer 208 provided between the semiconductor substrate 202 and the epitaxial layer 204; an n+ type sinker 210 which is connected to the impurity buried layer 208, and is provided on the surface of the semiconductor layer 206; an n− type deep well 212 which is formed on the surface of the semiconductor layer 206 in a region enclosed by the sinker 210; a p type base region 214 formed in the deep well 212; a p+ type base extraction region 218 and an n+ type emitter region 220, which are formed in the base region 214; and an n+ type collector extraction region 222 formed in the sinker 210.
In the bipolar transistor, a breakdown is caused and a current flows (dashed line (OFF) in the drawing) into the base extraction region 218 from the collector extraction region 222 through the deep well 212 on the surfaces of the semiconductor layer 206 and the base region 214, when a voltage is applied to the collector and the collector voltage reaches a avalanche breakdown voltage between the collector and the bases in the bipolar transistor. When the above current is increased, a potential just under the emitter region is increased by a component of a (current×resistance), based on a resistance component in the base region. A junction between the emitter and the base is turned on by the above increased potential, and current flows from the base to the emitter. Thereby, the bipolar transistor is operated, and a current flows (dashed line (ON) in the drawing) from the collector extraction region 222 to the emitter region 220 through the sinker 210, the impurity buried layer 208, the deep well 212, and the base region 214.
A bipolar transistor having the above configuration has been described in Japanese Laid-Open patent Application publication NO. 2003-224252, Japanese Laid-Open patent publication NO. 2002-26161, and Japanese Laid-Open patent publication NO. H02-102541.
The present inventor has recognized as follows. It has been difficult in a conventional structure to raise an off-breakdown-voltage to, for example, a voltage exceeding 100 V. The reason is that it is required to increase the density of the base region 214 in order to prevent punch-through between the emitter and the collector, and to raise the off-breakdown-voltage, though the increase of the density of the base region 214 reduces the avalanche voltage. Moreover, a conventional structure has had a problem that the characteristics of a bipolar transistor are changed when the bipolar transistor is used for a long time. The reason is that a breakdown is generated comparatively near the surface of a substrate.